In view of the fact that the friction properties of sheet metal play a determinant part during forming, such as during deep-drawing, it is important to take account of these properties during the forming of the sheet. Because of the varying surface pressures which arise during forming, it is necessary for the friction properties to be determined as a function of different surface pressures. In practice, this has been done hitherto in such a way that different sheet samples have been drawn one after another through gripping jaws at different surface pressures in each case. From the different surface pressures and the drawing force required for the drawing, the friction property of each sample of sheet is determined. It is clear that such a process is time-consuming because of the need to clamp the samples between the jaws one after another.
The objective on which the invention is based is to develop a process for the determination of the friction properties of metal sheets during forming, with which the friction properties can be determined with a broad spectrum of surface pressures.
In addition to this, the invention is based on the objective of creating a measuring apparatus for the performance of said process.
The invention accordingly relates to a process for the determination of the friction properties of sheet metal during its forming, whereby a sample of sheet metal is drawn at a given surface pressure between gripping jaws, and the drawing force necessary for the drawing is measured. Such a process is characterised according to the invention by the fact that, when the sample is drawn at a given drawing speed, the surface pressure is controlled according to a specified characteristic curve, and the friction properties dependent on the surface pressure in each case are determined from the values for the surface pressure and the drawing force assigned to one another.
With the process according to the invention, it is possible for the friction properties of a number of sheet samples to be determined rapidly in one procedure over a broad spectrum of surface pressure. It would be conceivable for the determination also to be made of the drawing speed or its incorporation, if it is varied. By contrast with the known process, with which individual sheet samples are drawn in each case with constant surface pressure through the gripping jaws, with the process according to the invention the phases which would otherwise be conventional, of the transition from adherence friction to slide friction, can be omitted, which also has a favourable effect on the measuring time required, while not impairing the indication significance of the measurement result.
In principle it is possible for the drawing of the metal sample through the gripping jaws to be effected with a drawing speed in accordance with a specified characteristic speed curve.
The change in the surface pressure can be effected in stages, as well as continuously.
A measuring device for the performance of the process is characterised by
a) A pair of gripping jaws for clamping a sample of sheet metal,
b) A pressure generator for the gripping jaws, the pressure force of which is adjustable,
c) A drawing device which grips the sample, with which the sample can be drawn through the gripping jaws at a specified drawing speed,
d) A control device for the pressure generator and/or a drive unit for the drawing device, with which the pressure force when drawing the sheet metal sample through the clamping jaws can be varied, and
e) A measurement and evaluation device, which determines the friction properties from the measurement values allocated to the surface pressure of the gripping jaws and the drawing force taking effect on the sheet sample over the drawing path, and, if appropriate, also the drawing speed.
For preference, the control device consists of a regulator for the pressure generator and, if appropriate, for the drive unit of the drawing device.